The present invention relates to syntactic foams and composites, as well as methods for preparing the same and uses for the syntactic foams and composites. More particularly, the present invention relates to syntactic foams and composites which contain aerogels.
Syntactic foam is generally considered prefabricated, manufactured “bubbles” or microsphere fillers in a resin milieu. Syntactic foams are composite materials whose resinous matrix is embedded with preformed particles such as glass or ceramic microspheres. Syntactic foams distinguish themselves from other foams by the fact that hollow or solid spheres of a predetermined size and packing composition are used to control the density of the foam.
Syntactic foams have been used for purposes which require a low density (mass per unit volume) packing material such as undersea/marine equipment for deep-ocean current-metering, anti-submarine warfare, sandwich composites, the aerospace industry and the automotive industry.
Examples of syntactic foams include for example U.S. Pat. No. 5,120,769 which relates to syntactic foams having an insoluble matrix, and U.S. Pat. No. 3,832,426 which relates to foam having an insoluble matrix and carbon microspheres. Syntactic foams having a soluble polymer matrix are mentioned in U.S. Pat. No. 5,432,205. Syntactic foams have many industrial applications.
Prior to syntactic foams, there were generally two types of foams: blown foams created by the injection of gas; and, self-expanding foams created through the use of chemicals. More recently, materials created by mixing a solid with minute spheres of glass, ceramic, or polymer are finding an increasing range of uses in industrial and high-tech applications.
Blown foams are made by, mixing or injecting a gas into a liquid and causing it to froth like soap bubbles in a bathtub. When the bubbles solidify, a foam is created. Typically, self-expanding foams require the use of at least two chemical constituents: one to decompose into a gas to form the bubbles and one to form the walls of the cells. Again, when the chemical constituent around the bubbles solidifies, a foam is created.
Rigid foams and processes for their production are well known in the art. Such foams are typically produced by reacting a polyisocyanate with an isocyanate reactive material such as polyol in the presence of a blowing agent. A lot of the blowing agents used in the past are no longer acceptable, and the ones developed in recent years are available at much higher costs. Furthermore, the state of the art rigid foams prepared with blowing agents do not exhibit the high compression strength required when foams are used, i.e. in applications like deep sea pipeline insulation, up to 10,000 feet or higher.
In recent years, the substantial increases in costs of the basic materials used to make foam, has encouraged the development and use of filler materials to reduce the amount of the basic materials used and the weight of the finished materials. One of the suggested filler materials and insulating materials utilizes hollow microspheres. The expression “syntactic” as used herein refers to the use of hollow spheres or other material in a polymer matrix to produce a cellular material.
Expanded microspheres having a synthetic thermoplastic resin shell that encapsulates a liquid blowing agent are known. See, for example, U.S. Pat. Nos. 4,829,094, 4,843,104 and 4,902,722. U.S. Pat. Nos. 4,829,094 and 4,843,104 relate to a syntactic-polymer foam composition having a low density filler containing free flowing microspheres.
U.S. Pat. No. 4,916,173 relates to a polyurethane (PU) syntactic foam composition for millable modeling stock applications. These PU syntactic foam compositions have high glass transition temperatures and low coefficients of thermal expansion, and are prepared from a polymeric isocyanate, an amine-based polyol, a polyether triol, molecular sieve material and hollow microspheres. The foams are described as a solid polymer matrix. These compositions are based on polymethylene poly(phenyl isocyanate) and result in low physical properties (i.e. tensile strength, elongation, etc.) which may be suitable for modeling stock applications, but not for the more demanding requirements in deep sea pipeline insulation.
A solid polymer matrix is mentioned in U.S. Pat. No. 4,959,395. This patent relates to bulk polymerization of cycloolefin monomers by ring-opening polymerization wherein the microencapsulated blowing agents aid in filling molds during injection molding procedures such that both surfaces of the article being molded remain in contact with the mold surfaces.
U.S. Pat. Nos. 4,303,729 and 4,303,736 relate to the use of hollow plastic microspheres as filler materials in plastics. The microspheres described by these two are generally large diameter microspheres, i.e. in the range of 200 to 10,000 microns. These microspheres can be made from low thermal conductivity plastic compositions and blown with a low thermal conductivity gas to make improved insulation materials and composites.
Hollow microspheres having loadings of 2 to 5% by weight of the total composition are mentioned in U.S. Pat. No. 4,038,238. Low density polyurethanes are produced from rapid-setting polyurethane-forming compositions containing light weight hollow spheres or microballoons and a liquid viscosity reducing agent.
A rigid syntactic foam comprising glass microballoons is mentioned in U.S. Pat. No. 4,082,702. These foams are obtained by mixing an organic polyol, a polyisocyanate, a catalyst for the reaction of the polyol and the polyisocyanate, microballoons, and a flame retardant foam having a bimodal cell structure.
U.S. Pat. No. 3,510,392 relates to glass nodules in cellular polyurethane. The polyurethane contains a polyol and/or polyester reacted with an polyisocyanate, and water during crosslinking to provide a gaseous blowing agent. The reactive components are homogeneously mixed in a suitable mixing device with a surfactant and catalyst to control the rate of reaction. Cellulate glass nodules are added to the homogeneous mixture in the bottom of a mold cavity which is then closed and foaming occurs. These are suitable for building panels having a continuous polyurethane phase and a discontinuous phase (i.e. cellular glass nodules).
U.S. Pat. No. 6,166,109 relates to syntactic rigid PUR/PIR foam boardstock. These hollow microspheres are filled with a hydrocarbon, air or vacuum, to introduce uniform cell geometries in the foams. The microspheres, which have an average diameter of 0.01 to 60 microns, are encapsulated with a closed cell polyurethane foam. Foams in the examples are based on a polyester, a surfactant, catalysts, water, a chlorofluorocarbon blowing agent and a polymethylene poly(phenylisocyanate). These syntactic rigid foams have a bimodal cell structure.
JP 4257429 relates to the manufacture of foam sheets with smooth surfaces which are useful for thermal insulators and packaging materials. The foam sheets of this reference can be prepared by applying a composition containing an organic polymer binder and a low boiling point solvent sealed thermally expandable microcapsules on a base film, laminating a polyester film on the coated layer, heating to dry and expand the coated layer and removing the polyester film. The resultant foam sheets have uniform closed cells and a smooth surface.
Thermally insulating syntactic foam compositions are mentioned in U.S. Pat. No. 6,284,809. These foam compositions have thermal conductivities less than 0.120 watts/meter-° K and exhibit acceptable strength and buoyancy characteristics for sub-sea applications at depths of up to about 10,000 ft.
Conventional syntactic foams use prefabricated or manufactured “bubbles” such as microspheres. Some refer to the microspheres as microballoons or even macroballoons. Syntactic foams can be prepared by mechanically combining the microspheres with a resin to form a composite material. Whereas blown and self-expanding foams and surfactant foams develop a fairly random distribution of gas pockets of widely varying sizes and shapes, the porosity of syntactic foams can be much more closely controlled by careful selection and mixing of the microspheres with the resin milieu. Syntactic foams can also be called assembled foams.
While ordinary foams are visibly porous, syntactic foams can have cells so small that the material appears to be a homogeneous solid. Syntactic foams are typically used in deep-submergence vehicles, instrument packaging, electronic gear, cable buoys, floatation collars for deep-water drilling operations, radio frequency and aerospace applications, and by pattern-makers in factories. In other words, the foams are used in industrial applications where, for example, buoyancy is important. Syntactic foams can also be used as carriers of coated or uncoated chemicals, biologicals, nutraceuticals, growth factors, amino acids, bioactive materials and pharmaceutically active materials for pharmaceutical, sanitary, veterinary, agricultural and medical applications.
Some previous patents in this area include U.S. Pat. No. 3,856,721 relating to a syntactic foam produced by a controlled curing of a polymer which is a homopolymer of butadiene or a copolymer of butadiene and styrene or the like, at least 40% of which polymer is butadiene. Instead of styrene, a methyl or ethyl derivative can be used. The syntactic foam includes minute hollow spheres which give strength to the foam product and the syntactic foam product has a very low density. The polymeric material is subjected to a two-stage cure. The first stage being a low-temperature curing system utilizing methylethyl ketone (MEK) peroxide or other peroxides used in lower-temperature cures, cobalt naphthenate, iron naphthenate, and acetylacetone (pentanedione) or the like; the peroxide used in the second stage requiring a higher temperature for activation.
U.S. Pat. No. 4,250,136 relates to a sandwich of composite materials assembled and placed within a mold having the shape of the article to be formed. The composite sandwich is comprised of the following ingredients: (1) a first or bottom layer of reinforcing material such as fiberglass in woven or mat form; (2) a first layer of initially resilient and open-cell foam containing a liquid thermosetting resin such as epoxy, polyester, vinylester, or the like, is laid over the first reinforcing layer, (3) a second layer of reinforcing material is laid over the first resin-containing, open-cell foam layer; (4) a suitable quantity of uncured syntactic foam having a dough-like consistency is placed over the second reinforcing layer, (5) a third reinforcing layer is placed over the uncured and amorphous syntactic foam; (6) a second layer of liquid, resin-containing, open-cell, resilient foam is overlaid on the third reinforcing layer; and (7) a fourth or upper layer of reinforcing material is laid upon the second resin-containing foam layer. The composite sandwich is then placed within the mold and subjected to suitable heat and pressure to cause the uncured sandwich to assume the internal shape of the mold.
U.S. Pat. No. 4,425,441 relates to a high temperature and flame resistant closed cell polyimide foam material and methods of making the foam. An aromatic tetracarboxylic acid dianhydride is reacted with an oxontine to produce an N-substituted imide, which is then esterified with a suitable alcohol. The resulting liquid is dried and the dry residue is reduced to a uniform powder having particles with diameters generally in the 0.5 to 10 mm range. The powder is preferably further dried, either before or after final size reduction, in a moderate vacuum at moderate temperature to remove any excess residual alcohol. The powder spontaneously expands to form a closed cell foam when heated to a temperature in the range of about 90° to 150° C. for a suitable period. When the powder is expanded in a closed mold, a consolidated, closed cell foam product results. When expanded in an unrestricted manner, closed cell “macroballoons” having average diameters between about 0.4 mm to 15 mm result.
U.S. Pat. No. 4,518,717 relates to methods of making low density modified polyimide/polyimide-amide foams and the resulting compositions. An N-substituted aliphatic imide is prepared by reacting a suitable aromatic dianhydride with a suitable oxime. A polyimide forming material is prepared by dissolving the N-substituted aliphatic imide in an esterifying solvent, then adding a suitable aromatic diamine. This material is dried to a powder. A suitable hydrated compound which is stable up to at least about 100° C. is mixed with the powder. A foam is then produced by heating the material to a reaction temperature for a period sufficient to produce a stable foam. The material melts, then spontaneously expands into a foam which becomes self supporting and cures to a resilient flexible foam. The addition of the hydrated compound is found to result in an exceptionally low density foam. Depending upon heating conditions, a polyimide, polyimide-amide or mixture thereof may be produced, resulting in foams having selectively variable physical properties.
U.S. Pat. Nos. 4,161,477, 4,183,838, and 4,183,839 relate to certain polyimide compositions which are flame resistant and useful as coatings and adhesives. The coating and adhesive compositions described in the above-mentioned patents are made by first preparing a suitable bisimide by reacting an aromatic tetracarboxylic acid dianhydride with a cyclic amide or oxime.
Difficulties have been experienced, however, in producing syntactic foams that have a density which is comparable to conventional foams. Typical densities of syntactic foams vary between 0.3 and 0.5 g/cm3, whilst conventional foams typically vary between 0.01 and 0.1 g/cm3. The density of syntactic foams has generally been restricted by the limited porosity of the foams. Porosity is a measure of the total void volume (e.g., air filled, gas filled, or the presence of a low density component) of the syntactic foam, and constitutes the sum of the void volume of the microspheres and the interstitial void volume. Using current methods of syntactic foam manufacture, the void volume provided by the microspheres is greater than the void volume provided by the interstitial spaces. Thus, the density of syntactic foams have been limited by the void volume of the microspheres. As such, the application of syntactic foams have been limited.
The patents and publications mentioned above and throughout the present application are incorporated in their entirety by reference and form a part of the present application.